Recently, precision optical apparatuses such as a digital camera and a video camera have been increased in performance and downsized remarkably. Due to the realization of high performance and downsizing, there is a demand for optical elements such as an optical lens that are downsized, highly integrated, and capable of high performance to be used in the optical apparatuses. Among the optical elements, an aspherical lens is taking a major role in optical designing because use of a single aspherical lens realizes aberration elimination that has heretofore been realized by using plural spherical lenses and enables the high integration of optical system. In order to realize higher integration, there is an increasing demand particularly for an aspherical lens having a high refraction low dispersion property.
In an optical element molding method, a precision press molding method wherein a press molding surface is used as it is without polishing or the like attracts attention. The precision press molding method is a method of press-molding an optical glass gob called pre-form at a temperature higher than a glass transition point (Tg) by using a metal mold having a highly precise molding surface of a desired shape, and the precision press molding method is an important molding method that enables bulk production of the aspherical lens.
A releasing film or the like is formed on a surface of the metal mold used in the precision press molding for the purpose of preventing adhesion of optical glass. Because deterioration of the releasing film is accelerated along with an increase in temperature and a film formation cost for the releasing film is high, there is a demand for an optical glass that can be molded at a low temperature, i.e., having a low glass transition temperature.
Boric acid-based glasses have heretofore been proposed as the optical glass having a low transition temperature in the field of high refraction (refractive index nd: 1.65 to 1.72) and low dispersion (Abbe number νd: 47 to 57) as disclosed in Patent Documents 1 and 2. However, the optical glasses disclosed in the publications have a problem of a lack in mass productivity due to its high liquid phase temperature (L.T.) and tendency to cause devitrification in the case of forming a pre-form thereof.
Though boric acid-based glasses obtained by improving the formerly mentioned boric acid-based glasses have been proposed by Patent Documents 3 and 4, the proposed optical glasses have a large linear thermal expansion coefficient and have a problem that an yield tends to be lowered since the glass is easily cracked due to heat shrinkage during press molding.
Patent Document 1: JP-A-8-26766
Patent Document 2: JP-A-5-201743
Patent Document 3: JP-A-2000-119036
Patent Document 4: JP-A-2000-16831